Cellular communication systems are known. Such systems are typically constructed with a number of partially overlapping service; coverage areas (cells) within which communication services may be provided to cellular radiotelephones (also commonly referred to as a communication units or mobile stations (MSs)) passing through such cells. Communication services within a cell are provided by a base transceiver station (BTS) typically located at a base site.
Cellular communication systems are typically organized with a number of BTSs located at a base site and under the control of a single base site controller (BSC). A BSC and associated BTSs are commonly referred to as a base station system (BSS). A number of BSSs are, in turn, controlled by a mobile switching center (MSC). The MSC in addition to routing calls among BTSs also provides in interconnect with a local public switch telephone network (PSTN) and to other MSCs.
A cellular system, covering a geographic area, allows an MS to communicate among other MSs or with public switch telephone network (PSTN) subscribers through the PSTN interconnect provided at the MSC. Calls received from an MS through a BTS are routed to the BSC which, in turn, routes the call request to the MSC. The MSC then routes the call to the desired party.
The BSC provides message transfer and call switching as directed by the MSC. Control information received from an MS through a BTS of the BSC are routed to the MSC. Calls received by a BSC from the MSC, on a channel of a communication link between the MSC and BSC, are switched under control of the BSC to the appropriate BTS in communication with the MS. Routing of all calls (including those between MSs under the same BSC) are routed through the MSC.
The BSC also typically provides a convenient location for transcoding voice information from a pulse coded format (PCM) used within the PSTN to a compressed version exchanged over the air interface. Speech compression may be accomplished within the transcoder using a linear predictive coding algorithm.
The MSC upon receipt of a call from an MS first determines whether the called party (target) is a MS under its control (within the MSCs geographic area). The MSC may make such a determination by reference to a call history stored within a memory of the MSC. If the target is a MS formerly within the MSC's geographic area, the MSC causes a page message to be generated for the target which is then transferred to each BSC covering the locale the MS was last known to be in. The BSCs then cause the page to be transmitted through each antenna of each BTS connected to the BSC. Upon identification of a location of a target the MSC issues switching commands, directed to requesting and target BSCs, providing a signal path therebetween.
If the MSC determines the target to be a PSTN subscriber, or a subscriber of another MSC (covering a different geographic area), then the MSC composes a call request message to be transmitted into a proper trunk group of the PSTN interconnect or a proper trunk group interfacing MSCs. Trunk groups, in general and channels within trunk groups in specific, are associated with specific geographic areas (and identity of target). An MSC determines the geographic area in which the target is located (and identifies whether the target is a PSTN subscriber or MS) by a translation of the called number. Upon identification of a type of target and geographic area the MSC transmits the call request into the trunk group associated with the target's identity and geographic area.
If the call request is received from the PSTN interconnect or from the MSC interconnect, then the MSC composes a page message for transmission through each BSS within the system. If the MS is within the system the MS responds through the nearest BTS. The response is transferred to the BSC, which in turn transfers the response to the MSC along with an identification of the receiving BTS. The MSC matches the call response with the call request and allocates a signal path from the interconnect to the BSC, along with instructions to the BSC to connect the selected channel of the receiving BTS to the allocated signal path.
Where a communication link has been established between a MS and target, and the MS moves from a first cell into a second cell (target cell), the communication link must be re-established through a BTS of the target cell or the call will be lost. As an aid in identifying the target cell, MSs typically measure signal strengths of signals from surrounding BTSs and transmit signal readings (and identities of a measured BTS) to a serving (source) BTS. The source BTS, in turn, transfers the readings to the MSC for evaluation. Where the target cell is served by the same MSC the MSC simply re-routes the call to the target BTS.
Where the target BTS is served by a second MSC (target MSC), the originating MSC (source MSC) must establish a communication path across a seam in the cellular communication system. The originating MSC does this by requesting access to the target BTS through a trunk group associated with the target MSC. The time required to re-route the call through the trunk group and target MSC results in an interruption of audio information that is perceived by a user as an audio hole or gap. Because of the importance of cellular communication a need exists for a means of re-routing calls between cells served by different MSCs that does not create audio gaps.